The present invention relates generally to a method of making plasma polymerized conjugated polymer films. More specifically, the present invention relates to making a plasma polymerized conjugated polymer film via plasma enhanced chemical deposition with a flash evaporated feed source of a low vapor pressure compound.
As used herein, the term xe2x80x9c(meth)acrylicxe2x80x9d is defined as xe2x80x9cacrylic or methacrylicxe2x80x9d. Also, (meth)acrylate is defined as xe2x80x9cacrylate or meth acrylatexe2x80x9d.
As used herein, the term xe2x80x9ccryocondensexe2x80x9d and forms thereof refers to the physical phenomenon of a phase change from a gas phase to a liquid phase upon the gas contacting a surface having a temperature lower than a dew point of the gas.
As used herein, the term xe2x80x9cconjugatedxe2x80x9d refers to a chemical structure of alternating single and double bonds between carbon atoms in a carbon atom chain.
The basic process of plasma enhanced chemical vapor deposition (PECVD) is described in THIN FILM PROCESSES, J. L. Vossen, W. Kern, editors, Academic Press, 1978, Part IV, Chapter IVxe2x80x941 Plasma Deposition of Inorganic Compounds, Chapter IVxe2x80x942 Glow Discharge Polymerization, herein incorporated by reference. Briefly, a glow discharge plasma is generated on an electrode that may be smooth or have pointed projections. Traditionally, a gas inlet introduces high vapor pressure monomeric gases into the plasma region wherein radicals are formed so that upon subsequent collisions with the substrate, some of the radicals in the monomers chemically bond or cross link (cure) on the substrate. The high vapor pressure monomeric gases include gases of CH4, SiH4, C2H6, C2H2, or gases generated from high vapor pressure liquid, for example styrene (10 torr at 87.4xc2x0 F. (30.8xc2x0 C.)), hexane (100 torr at 60.4xc2x0 F. (15.8xc2x0 C.)), tetramethyldisiloxane (10 torr at 82.9xc2x0 F. (28.3xc2x0 C.) 1,3,dichlorotetra-methyldisiloxane) and combinations thereof that may be evaporated with mild controlled heating. Because these high vapor pressure monomeric gases do not readily cryocondense at ambient or elevated temperatures, deposition rates are low (a few tenths of micrometer/min maximum) relying on radicals chemically bonding to the surface of interest instead of cryocondensation. Remission due to etching of the surface of interest by the plasma competes with the reactive depostion. Lower vapor pressure species have not been used in PECVD because heating the higher molecular weight monomers to a temperature sufficient to vaporize them generally causes a reaction prior to vaporization, or metering of the gas becomes difficult to control, either of which is inoperative.
The basic process of flash evaporation is described in U.S. Pat. No. 4,954,371 herein incorporated by reference. This basic process may also be referred to as polymer multi-layer (PML) flash evaporation. Briefly, a radiation polymerizable and/or cross linkable material is supplied at a temperature below a decomposition temperature and polymerization temperature of the material. The material is atomized to droplets having a droplet size ranging from about 1 to about 50 microns. An ultrasonic atomizer is generally used. The droplets are then flash vaporized, under vacuum, by contact with a heated surface above the boiling point of the material, but below the temperature which would cause pyrolysis. The vapor is cryocondensed on a substrate then radiation polymerized or cross linked as a very thin polymer layer.
The material may include a base monomer or mixture thereof, crosslinking agents and/or initiating agents. A disadvantage of the flash evaporation is that it requires two sequential steps, cryocondensation followed by curing or cross linking, that are both spatially and temporally separate.
According to the state of the art of making plasma polymerized films, PECVD and flash evaporation or glow discharge plasma deposition and flash evaporation have not been used in combination. However, plasma treatment of a substrate using glow discharge plasma generator with inorganic compounds has been used in combination with flash evaporation under a low pressure (vacuum) atmosphere as reported in J. D. Affinito, M. E. Gross, C. A.. Coronado, and P. M. Martin, A Vacuum Deposition Of Polymer Electrolytes On Flexible Substrates. xe2x80x9cPaper for Plenary talk in A Proceedings of the Ninth International Conference on Vacuum Web Coatingxe2x80x9d, November 1995, ed R. Bakish, Bakish Press 1995, pg 20-36., and as shown in FIG. 1a. In that system, the plasma generator 100 is used to etch the surface 102 of a moving substrate 104 in preparation to receive the monomeric gaseous output from the flash evaporation 106 that cryocondenses on the etched surface 102 and is then passed by a first curing station (not shown), for example electron beam or ultra-violet radiation, to initiate cross linking and curing. The plasma generator 100 has a housing 108 with a gas inlet 110. The gas may be oxygen, nitrogen, water or an inert gas, for example argon, or combinations thereof. Internally, an electrode 112 that is smooth or having one or more pointed projections 114 produces a glow discharge and makes a plasma with the gas which etches the surface 102. The flash evaporator 106 has a housing 116, with a monomer inlet 118 and an atomizing nozzle 120, for example an ultrasonic atomizer. Flow through the nozzle 120 is atomized into particles or droplets 122 which strike the heated surface 124 whereupon the particles or droplets 122 are flash evaporated into a gas that flows past a series of baffles 126 (optional) to an outlet 128 and cryocondenses on the surface 102. Although other gas flow distribution arrangements have been used, it has been found that the baffles 126 provide adequate gas flow distribution or uniformity while permitting ease of scaling up to large surfaces 102. A curing station (not shown) is located downstream of the flash evaporator 106.
In the flash evaporation process using acrylate and/or methacrylate the starting monomer is a (meth)acrylate monomer (FIG. 1b). When R1 is hydrogen (H), the compound is an acrylate and when R1 is a methyl group (CH3), the compound is a methacrylate. If the group R2 pendant to the (meth)acrylate group is fully conjugated, the Oxe2x80x94Cxe2x80x94 linkage interrupts the conjugation and renders the monomer non-conducting. Exposure to electron beam radiation, or UV in the presence of a photoinitator, initiates polymerization of the monomer by creating free radicals at the (Cxe2x95x90C) double bond in the (meth)acrylate linkage. After polymerization, the two (meth)acrylate Double (Cxe2x95x90C) bonds, where the cross-linking occurred, have been converted to single (Cxe2x80x94C) bonds. Thus, the cross-linking step further interrupts the conjugation and makes conductivity impossible.
Therefore, there is a need for an apparatus and method for making plasma polymerized conjugated polymer layers at a fast rate but that is also self curing, preserving the conjugation.
The present invention is an improved method of plasma polymerization wherein a conjugated monomer is cured during plasma polymerization.
The present invention may be viewed from two points of view, vis (1) an apparatus and method for plasma enhanced chemical vapor deposition of conjugated low vapor pressure monomer or a mixture of monomer with particle materials onto a substrate, and (2) an apparatus and method for making self-curing conjugated or conductive polymer layers, especially self-curing PML polymer layers. As used herein, the term xe2x80x9cconjugated polymerxe2x80x9d or xe2x80x9cfully conjugated polymerxe2x80x9d is defined as a polymer having sufficient degree of conjugation to be electrically conductive when doped. Thus, either the monomer is fully conjugated or the particles either combine together or crosslink with the monomer in a manner to provide a xe2x80x9cfully conjugated polymerxe2x80x9d.
From both points of view, the invention is a combination of flash evaporation with plasma enhanced chemical vapor deposition (PECVD) that provides the unexpected improvements of permitting use of low vapor pressure monomer conjugated materials in a PEDVD process and provides a self curing from a flash evaporation process, at a rate surprisingly faster than standard PECVD deposition rates.
Generally, the apparatus of the present invention is (a) a flash evaporation housing with a monomer atomizer for making monomer droplets, heated evaporation surface for making an evaporate from the monomer droplets, and an evaporate outlet, (b) a glow discharge electrode downstream of the evaporate outlet for creating a glow discharge plasma from the evaporate, wherein (c) the substrate is proximate the glow discharge plasma for receiving and cryocondensing the glow discharge plasma thereon. All components are preferably within a low pressure (vacuum) chamber.
The method of the present invention has the steps of (a) flash evaporating a liquid conjugated monomer an evaporate outlet forming an evaporate; (b) passing the evaporate to a glow discharge electrode creating a glow discharge conjugated monomer plasma from the evaporate; and (c) cryocondensing the glow discharge conjugated monomer plasma on a substrate whereupon condensed glow discharge conjugated monomer plasma as a liquid begins crosslinking. The crosslinking results from radicals created in the glow discharge plasma and achieves self curing.
It is an object of the present invention to provide a method of making a conjugated or conductive polymer.
An advantage of the present invention is that it is insensitive to a direction of motion of the substrate because the deposited conjugated monomer layer is self curing. A further advantage is that the conjugation is preserved during curing. In the prior art, the deposited monomer layer required a radiation curing apparatus so that the motion of the substrate had to be from the place of deposition toward the radiation apparatus and which interfered with conjugation as previously discussed. Another advantage of the present invention is that multiple layers of materials may be combined. For example, as recited in U.S. Pat. No. 5,547,508 and 5,395,644, 5,260,095, hereby incorporated by reference, multiple polymer layers, alternating layers of polymer and metal, and other layers may be made with the present invention in the vacuum environment.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following detailed description in combination with the drawings wherein like reference characters refer to like elements.